1. Field of the Invention
The present invention relates to a device which picks up elastic waves, such as a geophone, which is electrically inclination compensated and therefore suited to work in different orientations (directions).
2. Description of the Prior Art
Geophones are designed to pick up low-frequency or very low-frequency vibrations. They most usually comprise a cylindrical moving coil suspended around a magnet by means of springs. The coil oscillates along an axis on either side of a position of equilibrium defined by the springs. The springs are necessarily very flexible in order to obtain a very low resonant frequency. They are suited to work in positions where the coil is either vertical or horizontal. The springs are designed to compensate for the weight of the coil and to keep it centered around the magnet. The characteristics of the springs are different in each case. Compensation is perfect only for a well-defined operating position. If the geophone is moved away from the operating position for which it is designed, the coil is no longer properly centered around the magnet, which leads to a harmonic distortion increase. The offset thus generated has the effect of reducing the maximum travel of the coil around its position of rest where it comes to a stop position and therefore of reducing the dynamic reproduction range of the geophone.
The unbalance is all the greater as the geophone moves away from its natural operating position. It is a real handicap for applications where the operating position of the geophone is difficult to control, which is notably the case in deviated wells. A known solution consists in mounting the geophones in a double-gimballed and unbalanced suspension holding the oscillating system in its optimum position whatever the effective orientation of the housing. This solution is complicated and therefore expensive. It affects the response of the geophone in the upper part of the seismic frequency band. The geophone is bulky and difficult to fit in the generally limited spaces available.
Examples of the prior art in the field of geophones with moving coils are for example described in the following U.S. Pat. No. 4,811,211; 4,618,949, and 4,525,819.
A device for picking up elastic waves (seismic or acoustic waves) according to the invention is suited to work in positions that are far from the natural operating position thereof, for which it is designed, without the aforementioned drawbacks.
The device of the invention comprises an electrodynamic pickup including a magnetic source which generates a magnetic field, a moving coil placed in the magnetic field, and springs which hold the coil in a well-centered static position when the axis thereof is parallel to a reference direction (horizontal or vertical for example). The device comprises a current source which applies to the coil an electric current which corrects any unbalance of the coil in relation to its static position due to an inclination of the axis of the coil in relation to the reference direction.
The current source comprises a source of direct current and a voltage divider whose division factor depends on the angle between the axis of the geophone and a reference axis.
The divider comprises for example a potentiometer whose cursor is connected to an unbalance sensor.
According to another embodiment, the current source comprises a distortion analyzer which generates the electric correction current suited to minimize the distortion.